Lantibiotics are peptides having antibiotic and other activities, produced by Gram-positive bacteria. They contain, among other modified residues, the thioether amino acids lanthionine and methyllanthionine, which cross-link the peptide chain into a polycyclic structure. They have been classified into two classes, type-A and type-B, though such classification is not unproblematic. Type-A lantibiotics are generally elongate amphiphiles that are capable of forming pores in bacterial and other plasma membranes. Examples are nisin and subtilin. Type-B lantibiotics, by contrast, are globular, conformationally defined peptides that inhibit enzyme functions. Examples are cinnamycin and duramycin.
Activities ascribed to type-B lantibiotics such as cinnamycin include antimicrobial activity (providing potential application as antibiotics), inhibition of angiotensin-converting enzyme (providing a potential application in blood pressure regulation), immunomodulation via inhibition of phospholipase A2 (providing a potential application as anti-inflammatories), and interference with prostaglandin and leucotriene biosynthesis.
Type-B lantibiotics appear to exert their activity by interfering with enzyme activities by blocking the respective substrates. For example, type B lantibiotics such as mersacidin and actagardine have been found to inhibit biosynthesis of peptidoglycan; transglycosylation was identified as the target reaction. The substrate for this reaction is the lipid-bound cell wall precursor lipid II. While this is a target for the lantibiotic vancomycin, the site of action is different and is a new target binding site not used by any current antibacterial drug.
For the cinnamycin class of type B lantibiotics antibacterial activity has been observed, in particular with Bacillus strains, with effects described on membrane functions, ATP-dependent proton translocation and Ca2+-uptake, and on ATPases. Also, the formation of defined pores in phosphatidylethanolamine-containing planar membranes has been reported. These effects can be attributed to the specific binding of these type-B lantibiotics to phosphatidylethanolamine.
Lantibiotics have been shown to have efficacy and utility as food additives and antibacterial agents against Propionibacterium acnes and problematic pathogens, e.g. methicillin-resistant Staphylococcus aureus (MRSA), which has or is developing resistance to many commonly used antibiotics, and Streptococcus pneumoniae. For reviews, see Sahl and Bierbaum (1998) Annual Rev. Microbiol. 52:41-79; Jack and Sahl (1995) TIBTECH 13:269 278; Gasson (1995) Chapter 10, Lantibiotics, in Vining and Stuttard (eds) Biotechnology Series: Genetics and Biochemistry of Antibiotic Production, Biotechnological I 30 Series 28, pages 283-306.
Within the field of antibiotics, there is a continuing need for the provision of new antibiotic compounds, to overcome issues such as resistance, bio-compatibility, toxicity and the like. Accordingly, methods of producing lantibiotics, and the production of variant forms of lantibiotics (which may have a different activity profile compared to native forms), are desirable.
Actagardine is a known type B tetracyclic lantibiotic, 19 amino acids in length (1890 Da). It has potent activity against important Gram positive pathogens such as Staphylococcus aureus and Streptococcus pyogenes both in vitro and in in vivo animal models. The structure of actagardine is shown in FIG. 4. The compound is produced from a pre-pro-peptide, the C-terminal portion of which has the polypeptide sequence of SSGWVCTLTIECGTVICAC (SEQ ID NO:4). The polypeptide of SEQ ID NO:4 is modified by the following crosslinks, creating secondary and tertiary structure: CROSSLINK 1-6, Lanthionine (Ser-Cys); CROSSLINK 7-12, Beta-methyllanthionine (Thr-Cys); CROSSLINK 9-17, Beta-methyllanthionine (Thr-Cys); CROSSLINK 14-19, Beta-methyllanthionine sulfoxide (Thr-Cys).
Actagardine has been reported to be produced by two species of Actinoplanes; A. garbadinensis and A. liguriae. Also co-produced is an analogue in which the CROSSLINK 14-19 is not oxidized i.e. it is a beta-methyllanthionine not betamethyllanthionine sulfoxide which is named herein deoxy-actagardine.
U.S. Pat. No. 6,022,851 describes the isolation of actagardine from isolated strains of A. garbadinensis and A. liguriae.